lua novelties 5.2

8/2/2019 Lua Novelties 5.2

1/28

The Novelties of Lua 5.2

Roberto Ierusalimschy

September 2011


2/28

Long list of changes

a myriad of small improvements

light C functions

emergency garbage collection

ephemeron tables

bitlib

yieldable pcall/metamethods

generational collector

goto statementnew ENV scheme

2/28


3/28

Light C Functions

C functions without upvalues are stored as simple values, without memoryallocation

3/28


4/28

Light C Functions

only possible due to change in environments

new internal type concept of type variant opens the door for other variants (e.g., non-collectable strings)

implemented as a single pointer to functioneliminate the need for lua_cpcall

saves a few bytes of memory standard libraries create almost 200 light functions

portable way to represent other C functions in Lua C standard allows conversions between different types of C functions

4/28


5/28

Emergency Garbage Collection

when memory allocation fails, collector does a complete collection cycleand then tries again

5/28


6/28

Emergency Garbage Collection

seems obvious, but implementation is tricky

Lua allocates memory in lots of places

everything must be properly anchored before any allocation

finalizers (__gc metamethods) postponed during emergency collection

6/28


7/28

Ephemeron Tables

break cycles in weak tables where values refer to their keys

typical example:

local mem = setmetatable({}, {__mode = "k"})

function newKfunc (o)local f = mem[o]

if not f then

f = function () return o end

mem[o] = f

end

return f

end

7/28


8/28

Ephemeron Tables

despite weak keys, entries may never be removed from mem. each key has a reference to it in its value values are not (and cannot be) weak

ephemeron table: value is only alive when its key is alive

implementation has a quadratic worst case but only for weird scenarios

8/28


9/28

bitlib

library with bitwise operations

9/28


10/28

bitlib

a most-wanted feature in Lua

far from straightforward main problem: numbers in Lua are double in particular, -1 is different from 0xffffffff

some differences from older libraries signed unsigned results overflows in shift/rotate negative shifts

future problem: 64-bit operations

10/28


11/28

Yieldable pcall/metamethods

programs in Lua 5.2 can yield inside a pcall, a metamethod, or a foriterator

11/28


12/28

Yieldable pcall/metamethods

another most-wanted featureplanned to be the main change for Lua 5.2

basic idea from Mike Pall long-jump over C functions and call them again when resuming lua_pcall lua_pcallk allows function to signalize whether it can

yield at each point

change from original implementation: resume calls a continuationfunction

instead of the same function that was interrupted continuation passed as argument to lua_pcallk

metamethods resume through extra code to complete execution ofinterrupted opcodes

12/28


13/28

Generational Collector

garbage collector can use the generational algorithm

13/28


14/28


15/28

goto

Lua 5.2 will include a somewhat conventional goto statement

15/28


16/28

goto

goto fits nicely with Lua philosophy of mechanisms instead ofpolicies very powerful mechanism easy to explain

allows the implementation of several mechanisms

continue, redo, break with labels, continue with labels, state machines,etc.

Yes, even break is redundant may be removed in the future not worth the trouble now

break does not need to be last statement in a block restriction in place to allow break label in the future restriction does not make sense for goto

16/28


17/28

goto implementation

quite simple for the VM small change to unify OP CLOSE and OP JMP

parser must keep pending gotos and visible labels

visibility rules

closing of upvalues

break implemented as goto break each loop followed by a virtual label ::break::

optimization for a common case:

if a == b then goto L end

NEQ a b EQ a b

JMP 1 JMP L

JMP L

17/28

I l?


18/28

Isnt goto evil?

The raptor fences arent out are they?

continuations are much worse dynamic and unrestricted goto basic idea: l = getlabel(), goto(l) labels are first-class values

yet nobody complains; it is cool to support continuations

is the problem with goto that they are too restricted?

demonized for years of abuse

18/28


19/28

New ENV scheme

Several parts

_ENV instead of dynamic environments any global name var replaced by _ENV.var main functions receive an upvalue named _ENV upvalue initialized with global table by default

no more fenv for functions

no more fenv for threads

simplification in the support for modules no more module function

19/28

f


20/28

setfenv

modules in general, and module in particular, were the mainmotivations for the introduction of dynamic environments andsetfenv in Lua 5.0.

module was never very popularsetfenv, on the other hand, became popular for toying with otherfunctions

setfenv runs against function encapsulation

20/28

ENV


21/28

ENV

the new scheme, with _ENV, allows the main benefit of setfenv witha little more than syntactic sugar main benefit being the power to encapsulate all global names of a

module inside a table

being a syntactic sugar, it is much simpler than old environments both implementation and proper understanding

it also allows a reasonable emulation of setfenv needs the debug library, which seems fit

as a bonus, it allows some nice tricks on its own

_ENV as a function argument setfenv bound to specific functions

21/28

E i t f C f ti d th d


22/28

Environments for C functions and threads

environments for threads frequently misunderstood only visible from C when loading a new function through pseudo-index for globals

environments for threads seldom used some few uses tricky to replace

environments for C functions easily replaced by upvalues

opened the door for light C functions

less fat in the language implementation and documentation

22/28

Mod les


23/28

Modules

no more module function

in general, less implicit things

modules must explicitly change their environment and return their

tables

modules do not create globals by default small problems with -l option for Lua stand-alone common use: local mod = requiremod

23/28

What we did not do


24/28

What we did not do

removal of coercions

macros

24/28

Automatic Coercion


25/28

Automatic Coercion

Very convenient to concatenate numbers with strings print("the value is " .. x)

Apparently convenient for things like print(fact(io.read())) function fact (n)

if n == 0 then return 1

else return n * fact(n - 1) end

end

Mostly useless for many other cases is it?

Somewhat complex

25/28

Macros


26/28

Macros

several nice solutions in the small: token filters, m4-style, etc.

main problem (seldom discussed): programming in the large

26/28

Macros in the large


27/28

Macros in the large

modularization what is the scope of a macro? how to preload macros for a load?

libraries providing macros

same library can provide both macros and functions? how to require a library? (a predefined macro require?)

how to precompile code? should all macro libraries be present? do macros vanish in precompiled code?

error messages

27/28

Conclusions


28/28

Conclusions

a few long-wanted features yieldable pcall/metamethods bitlib emergency collector

many small improvements

good clean up of the module system overdone in Lua 5.1

there are still things to be done

28/28

lua novelties 5.2

Documents